Spot-welding equipment is used to weld steel plates kept in contact with each other. FIG. 24 is a cross-sectional view illustrating the spot welding of steel plates 50. As shown in FIG. 24, spot-welding of the steel plates 50 is performed by sandwiching the plates contacting each other with a pair of electrodes 52, and by applying a given force to the electrodes 52 in the direction of the arrow to pressurize the steel plates 50.
A large current on the order of kA is then fed to the electrodes 52 while the pressurized state is maintained to melt the compressed portion of the steel plates 50 instantaneously by Joule heat, thus forming a molten body of a given diameter called nugget 54 (non-patent literature 1).
By the way, as automotive materials, ultra-high-tension steel plates have recently been used extensively for the spot welding in vehicle production lines to reduce weight of vehicles and ensure safety at the same time.
FIGS. 25A and 25B are plan views of samples used for a tensile test for measuring the spot-weld strength of high-tension steel plates. FIG. 25A is a superposed joint sample, and FIG. 25B is a cross joint sample. With the superposed joint sample shown in FIG. 25A, the two rectangular steel plates 50 are made to contact each other and spot-welded at the ends in the longitudinal direction. With the cross joint sample shown in FIG. 25B, two rectangular steel plates 50 are made to cross in a cross shape, and the intersection is spot-welded. A nugget 54 formed by welding is shown in an ellipse enclosed with a dotted line, and the arrows show the direction of force 56 applied in the tensile test.
It has been reported that in spot welding of high-tension steel plates, the tensile strength of a superposed joint increases as the strength of the joint material increases, but that the peel strength of a cross joint does not increase with the improvement of material strength, meaning that it is difficult to obtain stable strength. The reason why stable tensile strength cannot be obtained under the peeling load applied to the cross joint is assumed to be simultaneous occurrence of the following two phenomena: the degree of stress concentration on the circumference of the nugget 54 becomes extremely high, and the binding force around the nugget 54 increases with the increase in strength of the parent metal. Under such circumstances, to ensure the toughness of the weld region of high-tension steel plates for actual vehicle bodies, compositional control is performed at present. For example, the amount of carbon is maintained at a certain level or lower to prevent the weld region from becoming too hard.
Meanwhile, since the use of high-tension steel plates is an efficient way to reduce the weight of vehicle bodies, high-tension steel plates with enhanced strength and ductility are much anticipated. By further improving the strength of steel plates for vehicles, further reduction in weight is expected. By improving the ductility of steel plates for vehicles, press-molding performance can be improved and sufficient deformability at possible collision in the state of products are ensured. Normally, the higher the strength of steel plates for vehicles, the lower the ductility. To improve the strength and ductility of steel plates for vehicles at the same time, it is effective to increase the carbon content in the material. In this case, however, the spot-weld region becomes extremely hard and brittle, and thus it is difficult to obtain desired strength stably.
Various efforts have been made to ensure higher strength of spot-weld regions by improving welding methods. For example, post-energization for tempering is attempted after a weld joint is formed in a given size. However, with the resistance spot welding performed in vehicle assembly, the process time per stroke must be kept within one second. Consequently, if tempering is performed by post-energization using existing welding equipment, the effect of tempering becomes minimal. Or, to obtain sufficient effect of tempering, the time for tempering far exceeds the specified process time. This problem is derived from the basic problem of resistance welding that efficient heating cannot be caused in a short time because the current density of the welded region decreases as the energized area increases after the formation of a nugget 54.
Patent literature 1 discloses spot welding equipment having a spot welder and a high-frequency induction heating means to improve the fatigue strength of the spot-welded region of high-tension steel plates. This high-frequency induction heating means includes a heating coil for heating the portion of the work to be welded by induction heating and a high-frequency power source for supplying high-frequency power to the heating coil.